Method of producing oxocylohexyl or oxocyclohexylene derivatives

ABSTRACT

The invention relates to a method for producing compounds of general formula (I), wherein R t  represents an oxocyclohexyl radical which is optionally substituted by a hydroxyl radical, an alkoxy radical, and/or alkyl radicals, or an oxocyclohexenyl radical which is optionally substituted by a hydroxyl radical, an alkoxy radical, and/or alkyl radicals, the oxo groups being in the form of an acetal, a ketone, or an enol ether, and R 2  represents a hydroxy group or a protective group which can be transformed into a hydroxy group by means of hydrolysis. A compound of general formula (II) is cathodically reduced in a divided or partially divided electrolysis cell at a pH value of between 2 and 9.

[0001] The present invention relates to a process for the preparation of compounds of the formula I

[0002] in which R¹ is an oxocyclohexyl radical optionally substituted by a hydroxyl radical, an alkoxy radical and/or alkyl radicals or an oxocyclohexenyl radical optionally substituted by a hydroxyl radical, an alkoxy radical and/or alkyl radicals, where the oxo groups can be present in the form of an acetal, ketone or enol ether, and

[0003] R² is a hydroxyl group or a protective group convertible by hydrolysis into a hydroxyl group,

[0004] in which a compound of the general formula II

[0005] in which R¹ and R² in each case have the same meaning as in the general formula I,

[0006] is reduced cathodically in a divided or quasi-divided electrolysis cell at a pH of from 2 to 9.

[0007] A multiplicity of the industrial carotenoid syntheses described in the literature, inter alia the preparation of astaxanthin, proceed via cyclohexene intermediates which, in addition to one or more C—C double bonds, also contain a C—C triple bond. For the formation of a conjugated double bond system, this triple bond must be partially reduced in a separate process step.

[0008] In the astaxanthin synthesis described in DE-A 43 22 277 and the previously unpublished DE-A-10049271, the preparation of specific compounds of the formula I from compounds of the general formula II is carried out by reaction with zinc/acetic acid in dichloromethane.

[0009] In EP-A 0 005 748, a further process for the partial reduction of an alkynediol, likewise with zinc/acetic acid, is described.

[0010] The disadvantage of the zinc/acetic acid method is the inadequate selectivity. Side reactions, for example the formation of spiro compounds, which in the further course of the synthesis cannot be converted into the desired secondary products, can lead to marked losses in yield.

[0011] For the electrochemical reduction of alkynes without functional groups, e.g. aliphatic or araliphatic hydrocarbon compounds having a C—C triple bond, such as phenylacetylene, numerous methods are described, i.e. for example, an electrolysis in 10% strength sulfuric acid in ethanol on nickel cathodes (J. Am. Chem. Soc. 1943, 965).

[0012] In EP-A-0 085 763, an electrochemical process for the reduction of alkynediols is described. Electrolysis is carried out on lead electrodes in a basic solution in a divided electrolysis cell.

[0013] As the comparison example shows, the reaction conditions for the present compounds are not suitable, since decomposition reactions occur under the alkaline conditions.

[0014] It is an object of the present invention to make available an electrochemical process for the preparation of the compounds as defined in high yields and with high selectivity, with which the abovementioned disadvantages of the prior art can be avoided and which allows a reaction procedure which is as simple as possible.

[0015] We have found that this object is achieved by the process described at the outset.

[0016] The alkoxy radicals with which the oxocyclohexyl radicals or oxocyclohexenyl radicals can be substituted are preferably C₁- to C₄-alkoxy radicals.

[0017] The alkyl radicals with which the oxocyclohexyl radicals or oxocyclohexenyl radicals can be substituted are preferably C₁- to C₄-alkyl radicals, particularly preferably methyl radicals.

[0018] If the oxo groups are present in the acetal form, they are preferably derived from primary C₁- to C₆-monoalkyl alcohols or diprimary C₁- to C₆-dialkyl alcohols. The same applies analogously for the case in which the oxo group is present in the form of an enol ether. In the case in which the oxygen atoms of the hydroxyl group and the enol ether group are bonded to vicinal C atoms, these two oxygen atoms can also be connected by the C₁- to C₄-alkylene unit which is optionally substituted by alkyl radicals.

[0019] Suitable protective groups for R² convertible into a hydroxyl group by hydrolysis are those functional groups which can be converted into a hydroxyl group relatively easily. Mention may be made, for example, of ether groups, such as benzyloxy groups and tert-butyloxy groups, silyl ether groups, such as —O—Si (CH₃)₃, —O—Si (CH₂CH₃)₃, —O—Si (i-propyl)₃, —O—Si (CH₃)₂(tert-butyl) und —O—Si (CH₃)₂(n-hexyl), or substituted methyl ether groups, such as the α-alkoxyalkyl ether groups of the formulae:

[0020] and suitable pyranyl ether groups, such as the tetrahydropyranyloxy group and the 4-methyl-5,6-dihydro-2H-pyranyloxy group.

[0021] The use of the tetrahydropyranyloxy group for

[0022] or the alpha-ethoxyethoxy ether group of the formula

[0023] is particularly preferred.

[0024] Conditions for the removal of these protective groups are described, for example, in T. Greene “Protective Groups in Organic Chemistry”, which appeared in Wiley-Verlag 1981.

[0025] The process according to the invention is particularly suitable for the preparqtion of compounds of the general formula I in which R¹ is radicals of the general formula IIIa, IIIb, IIIc or IIId

[0026] where R³, R⁴ and R⁵ independently of one another are hydrogen or optionally substituted C₁-C₄-alkyl.

[0027] Alkyl radicals for R³ and R⁴ which may be mentioned are linear or branched C₁-C₄-alkyl chains, e.g. methyl, ethyl, n-propyl, 1-methylethyl, n-butyl, 1-methylpropyl, 2-methylpropyl, and 1,1-dimethylethyl. Preferred alkyl radicals are methyl and ethyl.

[0028] The radicals R³ and R⁴ can also form a cycloheptyl or cyclohexyl ring together with the carbon atom to which they are bonded.

[0029] Substituents for R⁵ which may be mentioned are linear or branched C₁-C₄-acyl chains, e.g. formyl, acetyl, propionyl, isopropionyl. The preferred acyl radical is acetyl.

[0030] Using the process according to the invention, the compound of the formula Ia and the correpsonding cis isomers (below also for short: “cis-Asta-C15-EN” or “trans-Asta-C15-EN”) can be prepared particularly advantageously from the compound of the formula IIa (below also for short: “Asta-C15-IN”).

[0031] The preparation of the starting compounds of the general formula II is described, for example, in EP 633 258.

[0032] Suitable cathode materials are preferably lead, graphite or alternatively mixtures of lead and graphite. Furthermore, the following materials can be employed as cathode: zinc, copper, silver, tin, stainless steel, all conventional hydrogenation metals, in particular Co, Ni, Ru, Rh, Re, Pd, Pt, Os, Ir, and Cd. Ni, Co, Ag and Fe can be employed as Raney metals, which can optionally be doped by foreign metals such as Mo, Cr, Au, Mn, Hg, Sn or other elements of the Periodic Table, in particular S, Se, Te, Ge, Ga, P, Pb, As, Bi, or Sb.

[0033] Suitable anode materials are all customarily employed materials, preferably graphite, lead dioxide, platinum, oxygen-evolving DSA® anodes.

[0034] In the process according to the invention, the current densities are in general from 100 to 10 000 A/m², preferably from 300 to 5000 A/m².

[0035] The process according to the invention is generally carried out at temperatures from −10° C. up to the boiling point of the solvent used in each case, temperatures from 5° to 100° C., in particular from 5 to 30° C., being preferred.

[0036] Depending on the compound to be reacted, the process according to the invention can be carried out at a pH from 2 to 9, preferably at a pH from 3 to 8, particularly preferably at from 4 to 7.

[0037] In the context of the process according to the invention, the nature of the cell type used, the shape and the arrangement of the electrodes has an influence, so that in principle restrictions below must be observed.

[0038] a) Electrolysis in Divided Cells

[0039] Divided cells having a plane-parallel electrode arrangement are preferably used, since anolyte and catholyte must be separated from one another in order to be able to exclude in the course of the process according to the invention starting materials, like products, undergoing chemical side reactions due to the anode process. The separating media employed can be ion-exchange membranes, microporous membranes, diaphragms, filter fabric made of non electron-conducting materials, glass frits, and porous ceramics. Preferably, ion-exchange membranes, in particular cation-exchange membranes, are used. These conductive membranes are commercially obtainable, for example, under the trade names Naf ion® (E.T. DuPont de Nemours and Company) and Gore Select® (W. L. Gore & Associates, Inc.).

[0040] Preferably, the electrodes are arranged plane-parallel, as with this design a homogeneous current distribution is afforded with small electrode gaps, with two gaps of from 0.01 to 10 mm each, preferably from 0.01 to 3 mm, in the anodic and/or cathodic gap.

[0041] b) Undivided Cells or Quasi-Divided Electrolysis Cell

[0042] It has been shown that the present process can also be carried out in an undivided procedure if the surface of the working electrode (in this case the cathode) and the counterelectrodes (in this case the anode) differ greatly in their size. Preferably, the area of the anode is reduced to values from 1 to 50% of the cathode area, furthermore preferably to from 3 to 30% and particularly preferably to from 5 to 20%. These cell constructions are called pseudo-divided or quasi-divided.

[0043] Electrolysis cells consisting of a monopolar cathode, a monopolar or one or more intermediate bipolar electrodes are particularly preferred for this, where

[0044] the cathode and the parts of the bipolar electrodes charged in the same sense to this together form the working electrode and the anode and the parts of the bipolar electrodes charged in the same sense to this together form the counterelectrode

[0045] the space between counter- and working electrode is undivided

[0046] the surface of the counterelectrode consists of electrochemically active and inactive parts

[0047] the sum of the electrochemically active parts of the surface of the counterelectrode is smaller by a multiple than that of the electrochemically active parts of the surface of the working electrode.

[0048] In general, the electrolysis cell is designed as a stacked plate cell or capillary gap cell.

[0049] The material from which the anode (counterelectrode) is produced is in general selected from the following group: massive graphite, graphite board, massive metal, massive graphite covered on the electrolyte contact area with a thin layer of metal foil, massive graphite covered on the electrolyte contact area with a cation- or anion-exchange membrane which is optionally coated with a catalyst.

[0050] The material from which the cathode is produced is in general selected from the following group: massive graphite, graphite board, massive metal, graphite felt sheets, carbon felt sheets, fabric having a carbon-covered electrolyte contact area, porous solids, filled with carbon, porous metals, e.g. metal sponges.

[0051] The difference in the surfaces can be achieved, for example, by employing a material having a large surface area per volume such as graphite felt for the large-surface working electrode, while the counterelectrode consists of massive material having a relatively small surface area per volume such as graphite sheets. Furthermore, the difference in the electrode surfaces can be produced or increased by covering the counterelectrode partially with a nonconducting plastic film.

[0052] Electrolysis cells of this type and particularly preferred embodiments thereof are described in DE-A-10 063 195.

[0053] The electrochemical process according to the invention can be carried out either continuously or batchwise.

[0054] In general, the electrochemical process according to the invention is performed in the presence of an auxiliary electrolyte. In addition to the adjustment of the conductivity of the electrolysis solution, the auxiliary electrolyte occasionally also serves for controlling the selectivity of the reaction. This is particularly important in the present case, since the compounds to be reacted exhibit a strong pH-dependent stability.

[0055] As a rule, the content of the auxiliary electrolyte lies at a concentration of from 0.1 to 10, preferably from 0.2 to 3, % by weight, in each case based on the reaction mixture. Suitable auxiliary electrolytes are protonic acids, such as organic acids, e.g. sulfonic acids such as methylsulfonic acid, benzenesulfonic acid or toluenesulfonic acid, carboxylic acids such as benzoic acids, C₁-C₁₂-alkanoic acids, in particular acetic acid, mineral acids, such as sulfuric acid, hydrochloric, hydrobromic and hydriodic acid and phosphoric acid. Buffer solutions which can be prepared from the corresponding acids and their salts are particularly preferred, acetate buffer is particularly preferably used.

[0056] Furthermore, auxiliary electrolytes which can be used are also neutral salts. Suitable cations here are metal cations of lithium, sodium, potassium, cesium, but also tetraalkylammonium cations, such as, for example, tetramethylammonium, tetraethylammonium, tetrabutylammonium and dibutyldimethylammonium.

[0057] Anions which may be mentioned are: fluoride, tetrafluoroborates, sulfonates, such as, for example, methylsulfonate, benzenesulfonate, toluenesulfonate, sulfates, such as, for example, sulfate, methylsulfate, ethylsulfate, phosphates, such as, for example, methylphosphate, dimethylphosphate, diphenylphosphate, hexafluorophosphate, phosphonates, such as, for example, methylphosphonate methyl ester and phenylphosphonate methyl ester, but also the salts of the abovementioned organic acids, such as, for example, acetate or the halides chloride, bromide and iodide. Possible cations in these compounds are again the abovementioned cations.

[0058] The use of buffer systems in order to obtain a stability of the starting materials to be reacted and their products which is as high as possible is particularly preferred. Phosphate buffers and acetate buffers and mixtures of acetate buffers in combination with other conducting salts are preferred.

[0059] Suitable solvents in the process according to the invention are, in principle, all protic solvents, i.e. solvents which contain and can release protons and/or can form hydrogen bonds, such as, for example, water, alcohols, amines, carboxylic acids, etc., if appropriate also aprotic polar solvents such as, for example, THF, 1,2-dimethoxyethane, dioxane or mixtures of protic, aprotic solvents and/or water. Preferably, lower alcohols such as methanol, ethanol, isopropanol, n-propanol, n-butanol, sec-butanol, tert-butanol, ethers, such as, for example, diethyl ether, 1,2-dimethoxyethane, furan, THF, acetonitrile and dimethylformamide are employed here for maintenance of the conductivity, preferably a mixture of these solvents or furthermore preferably water as a mixture with these solvents in all possible mixing ratios.

[0060] Furthermore, it has been shown that it is advantageous for the reduction to work in a two-phase system, in particualr THF/water and dioxane/water mixtures have proven suitable here. The two-phase solution is formed, however, only by addition of the starting materials and/or salts.

[0061] Alternatively to the abovementioned alcohols, their carboxylic acids or amides can also be used. Carboxylic acids which are preferably employed are: formic acid, acetic acid, propionic acid and relatively long-chain branched and unbranched carboxylic acids, furthermore also sulfuric acid, hydrochloric, hydrobromic and hydriodic acid.

[0062] When carrying out the process, it has been shown, that in the divided and quasi-divided procedure, when using the same electrode materials, comparable results are obtained. Furthermore, it has been shown that the electrolysis proceeds particularly advantageously if water is present. This increases the selectivity, in the case of the divided procedure the lifetime of the membranes, and increases the conductivity, whereby the cell voltage can be kept at energetically favorable values.

[0063] A further advantage has proven to be the addition of a solvent which, with the starting material to be employed and the corresponding conductive salt, forms a two-phase mixture with water. Possible solvents are the solvents described above.

[0064] Preferably, the electrolysis solution used is a 5 to 50% strength by weight solution of the compound of the formula II in one of the abovementioned solvents, particularly preferably a 5 to 20% strength by weight solution of the compound mentioned in THF which is mixed with water which contains the conducting salt.

[0065] The subject of the invention will be explained in greater detail with the aid of the following examples:

[0066] A. Examples According to the Invention

[0067] 1. Divided Electrolysis Cell, Two-Phase in THF/Acetate Buffer pH 5

[0068] The cathode used was lead, the catholyte a mixture of 70.5 g of 1M sodium acetate buffer (pH 5) and 70.5 g of THF. 7.5 g of the compound of the formula IIa (also abbreviated below as “Asta-C15-IN”) were dissolved in the catholyte. The catholyte was present in two-phase form.

[0069] The anolyte used was 200 g of a 1% strength aqueous sulfuric acid solution which was combined with an oxygen-evolving anode (DSA).

[0070] The two cell compartments were separated from one another by an ion-exchange membrane (Nafion 324). Firstly, both cell compartments were filled and recirculated and the electrolysis was carried out at 25° C. and a current density of 14 MA/cm². After an amount of charge of 2.5 F was applied, the electrolysis was shut off.

[0071] The experimental analysis showed 33% of cis-Asta-C15-EN and 11% of trans-Asta-C15-EN, in total 44% of valuable product.

[0072] 2. Divided Electrolysis Cell, Two-Phase in Dichloromethane/Acetate Buffer pH 5

[0073] The cathode used was lead, the catholyte a mixture of 70.5 g of 1M sodium acetate buffer (pH 5), in which 1% tetrabutylammonium chloride had been dissolved, and 70.5 g of dichloromethane. 7.5 g of Asta-C15-IN were dissolved in the catholyte. The catholyte was present in two-phase form.

[0074] The anolyte used was 200 g of a 1% strength aqueous sulfuric acid solution which was combined with an oxygen-evolving anode (DSA). The two cell compartments were separated from one another by an ion-exchange membrane (Nafion 324). Firstly, both cell compartments were filled and recirculated and the electrolysis was carried out at 25° C. and a current density of 14 MA/cm². After an amount of charge of 2.0 F had been applied, the electrolysis was shut off.

[0075] The experimental analysis showed 12% cis-Asta-C15-EN (compound cis-Ia) and 28% trans-Asta-C15-EN (compound trans-Ia), in total 40% of valuable product.

[0076] 3. Divided Electrolysis Cell, Single-Phase in Dioxane/Acetate Buffer pH 5

[0077] The cathode used was lead, the catholyte a mixture of 70.5 g of 1M sodium acetate buffer (pH 5) and 70.5 g of dioxane. 7.5 g of Asta-C15-IN (IIa) were dissolved in the catholyte. A single-phase, homogeneous solution was obtained.

[0078] The anolyte used was 200 g of a 1% strength aqueous sulfuric acid solution which was combined with an oxygen-evolving anode (DSA).

[0079] The two cell compartments were separated from one another by an ion-exchange membrane (Nafion 324). Firstly, both cell compartments were filled and recirculated and the electrolysis was carried out at 25° C. and a current density of 14 MA/cm². After an amount of charge of 2.0 F had been applied, the electrolysis was shut off.

[0080] The experimental analysis showed 17% of cis-Asta-C15-EN (cis-Ia) and 8% of trans-Asta-C15-EN (trans-Ia), in total 25% of valuable product.

[0081] 4. Divided Electrolysis Cell, Two-Phase in THF/Acetate Buffer pH 5

[0082] The cathode used waszinc, the catholyte a mixture of 70.5 g of 1M sodium acetate solution (pH 9.3) and 70.5 g of THF. Furthermore, 1% ammonium chloride was added. 7.5 g of Asta-C15-IN were dissolved in the catholyte. The catholyte was present in two-phase form.

[0083] The anolyte used was 200 g of a 1% strength aqueous sulfuric acid solution which was combined with an oxygen-evolving anode (DSA).

[0084] The two cell compartments were separated from one another by an ion exchange membrane (Nafion 324). Firstly, both cell compartments were filled and recirculated and the electrolysis was carried out at 25° C. and a current density of 14 MA/cm². After an amount of charge of 4.5 F had been applied, the electrolysis was shut off.

[0085] The experimental analysis showed 4% of cis-Asta-C15-EN and 17% of trans-Asta-C15-EN, in total 21% of valuable product.

[0086] 5. Divided Electrolysis Cell, Two-Phase in THF/Acetate Buffer pH 5

[0087] The cathode used was lead, the catholyte a mixture of 70.5 g of 1M sodium acetate buffer (pH 5) and 70.5 g of THF. 7.5 g of Asta-C15-IN were dissolved in the catholyte. The catholyte was present in two-phase form. The anolyte used was 200 g of a 2% strength aqueous sulfuric acid solution which was combined with a lead dioxide anode.

[0088] The two cell compartments were separated from one another by an ion exchange membrane (Nafion 324). Firstly, both cell compartments were filled and recirculated and the electrolysis was carried out at 25° C. and a current density of 14 mA/cm². After an amount of charge of 2.0 F had been applied, the electrolysis was shut off.

[0089] The experimental analysis showed 48% of cis-Asta-C15-EN and 9% of trans-Asta-C15-EN, in total 57% of valuable product.

[0090] A comparable experiment in THF/potassium acetate buffer pH 5 showed 50% of cis-Asta-C15-EN and 8% of trans-Asta-C15-EN, in total 58% of valuable product.

[0091] 6. Divided Electrolysis Cell, Two-Phase in THF/Acetate Buffer pH 5

[0092] The cathode used was graphite, the catholyte a mixture of 70.5 g of 1M sodium acetate buffer (pH 5) and 70.5 g of THF. 7.5 g of Asta-C15-IN (IIa) were dissolved in the catholyte. The catholyte was present in two-phase form. The anolyte used was 200 g of a 2% strength aqueous sulfuric acid solution which was combined with a lead dioxide anode.

[0093] The two cell compartments were separated from one another by an ion exchange membrane (Nafion 324). Firstly, both cell compartments were filled and recirculated and the electrolysis was carried out at 25° C. and a current density of 14 mA/cm². After an amount of charge of 4.0 F had been applied, the electrolysis was shut off.

[0094] The experimental analysis showed 14% of cis-Asta-C15-EN (cis-IIa) and 16% of trans-Asta-C15-EN (trans-IIa), in total 30% of valuable product.

[0095] 7. Divided Electrolysis Cell, Two-Phase in THF/Acetic Acid

[0096] The cathode used was lead, the catholyte a mixture of 70.5 g of 5% strength aqueous acetic acid and 70.5 g of THF. 7.5 g of Asta-C15-IN (IVa) were dissolved in the catholyte. The catholyte was present homogeneously in single-phase form.

[0097] The anolyte used was 200 g of a 2% strength aqueous sulfuric acid solution which was combined with a lead dioxide anode.

[0098] The two cell compartments were separated from one another by an ion exchange membrane (Nafion 324). Firstly, both cell compartments were filled and recirculated and the electrolysis was carried out at 25° C. and a current density of 14 MA/cm². After an amount of charge of 2.0 F had been applied, the electrolysis was shut off.

[0099] The experimental analysis showed 34% of cis-Asta-C15-EN and 7% of trans-Asta-C15-EN, in total 41% of valuable product.

[0100] 8. Quasi-Divided Electrolysis Cell, Two-Phase in THF/Acetate Buffer pH 5

[0101] The cathode used was graphite, a platinum wire served as the anode.

[0102] The electrolyte was composed of a 9.5% strength solution of Asta-C15-IN in 30 g of dioxane and 30 g of a 1M sodium acetate buffer pH 5 in water.

[0103] The electrolysis was carried out at 25° C. and a current density of 14 mA/cm². After an amount of charge of 3 F had been applied, the electrolysis was shut off.

[0104] The experimental analysis showed 12% of cis-Asta-C15-EN and 18% of trans-Asta-C15-EN, in total 30% of valuable product.

[0105] B. Comparison Experiments

[0106] 1. Divided Electrolysis Cell, Single-Phase in Dioxane/Aqueous NaOH pH 13

[0107] The cathode used was lead, the catholyte a mixture of 70.5 g of 0.1M sodium hydroxide solution (pH 13), and 70.5 g of dioxane. 7.5 g of the compound of the formula IIa were dissolved in the catholyte. A single-phase, homogeneous solution was obtained.

[0108] The anolyte used was 200 g of a 0.1 M aqueous sodium hydroxide solution which was combined with a platinum anode.

[0109] The two cell compartments were separated from one another by an ion exchange membrane (Nafion 324). Firstly, both cell compartments were filled and recirculated and the electrolysis was carried out at 25° C. and a current density of 10 mA/cm². After an amount of charge of 2.0 F had been applied, the electrolysis was discontinued.

[0110] The experimental analysis showed no valuable product at all—a decomposition occurred in the alkaline medium.

[0111] 2. For Comparison. Divided Electrolysis Cell, Single-Phase in Dioxane/Aqueous NaOH pH 13

[0112] The cathode used was lead, the catholyte a mixture of 7.0.5 g of 0.1M sodium hydroxide solution (pH 13), and 70.5 g of dioxane. 7.5 g of Asta-C15-IN (IVa) were dissolved in the catholyte. A single-phase, homogeneous solution was obtained.

[0113] The anolyte used was 200 g of a 0.1 M aqueous sodium hydroxide solution which was combined with a platinum anode.

[0114] The two cell compartments were separated from one another by an ion exchange membrane (Nafion 417). Firstly, both cell compartments were filled and recirculated and the electrolysis was carried out at 25° C. and a current density of 5 mA/cm². After an amount of charge of 0.7 F had been applied, the electrolysis was discontinued.

[0115] The experimental analysis showed 1% of cis-Asta-C15-EN and 3% of trans-Asta-CI5-EN, and 4% of Asta-C15-IN as starting material. The main part of the Asta-C15-IN was converted into decomposition products. 

1-11. (Canceled)
 12. A process for the preparation of compounds of the formula I or I′

in which R¹ is an oxocyclohexyl radical optionally substituted by a hydroxyl radical, an alkoxy radical and/or alkyl radicals or an oxocyclohexenyl radical optionally substituted by a hydroxyl radical, an alkoxy radical and/or alkyl radicals, where the oxo groups can be present in the form of an acetal, ketone or enol ether and where in the case in which the oxygen atoms of the hydroxyl group and the enol ether group are bonded to vicinal C atoms, these two oxygen atoms are optionally connected by a C₁- to C₄-alkylene unit which is optionally substituted by alkyl radicals, or is a radical of the formula IIIa or IIIc,

in which R³, R⁴ and R⁵ independently of one another are hydrogen or optionally substituted C₁-C₄-alkyl and R⁵ is substituted C₁-C₄-alkyl, or the radicals R³ and R⁴ form a cycloheptyl or cyclohexyl ring together with the carbon atom to which they are bonded and R² is a hydroxyl group or a protective group convertible by hydrolysis into a hydroxyl group, in which a compound of the formula II

in which R¹ and R² in each case have the same meaning as in the formula I, is reduced cathodically in a divided or quasi-divided electrolysis cell at a pH of from 2 to
 9. 13. A process as claimed in claim 12, where the radical R¹ is an oxocyclohexenyl radical optionally substituted by a hydroxyl radical, an alkoxy radical and/or methyl radicals, in which the C—C double bond is in the alpha-position to the oxo group.
 14. A process as claimed in claim 12, where the radical R¹ is a radical of the formula IIIa, IIIb or IIId

and R⁵ independently of one another is hydrogen or C₁-C₄-alkyl.
 15. A process as claimed in claim 12, where the reduction is carried out in solution using a mixture of water and an inert organic solvent.
 16. A process as claimed in claim 12, where, as cathode material, lead, graphite, zinc, copper, silver, tin, stainless steel or mixtures of lead and graphite are used.
 17. A process as claimed in claim 12, where the reduction is carried out in a divided cell in which the anode and cathode space are separated from one another by an ion exchange membrane.
 18. A process as claimed in claim 12, where the cathodic reduction is performed in a quasi-divided electrolysis cell in which the size of the surface area of the anode (counterelectrode) is 1 to 50% of the surface area of the cathode (working electrode).
 19. A process as claimed in claim 18, where the cathodic reduction is performed in a quasi-divided electrolysis cell consisting of a monopolar cathode, a monopolar anode and one or more intermediate bipolar electrodes, where the cathode and the parts of the bipolar electrodes charged in the same sense to this together form the working electrode and the anode and the parts of the bipolar electrodes charged in the same sense to this together form the counterelectrode the space between counter- and working electrode is undivided the surface of the counterelectrode consists of electrochemically active and inactive parts the sum of the electrochemically active parts of the surface of the counterelectrode is smaller by a multiple than that of the electrochemically active parts of the surface of the working electrode.
 20. A process as claimed in claim 18, where the electrolysis cell is designed as a stacked plate cell or capillary cell.
 21. A process as claimed in claim 18, where the material from which the counterelectrode is prepared is selected from the following group: massive graphite, graphite board, massive metal, massive graphite covered on the electrolyte contact area with a thin layer of metal foil, massive graphite covered on the electrolyte contact area with a cation- or anion-exchange membrane which is optionally coated with a catalyst.
 22. A process as claimed in claim 18, where the material from which the working electrode is produced is selected from the following group: massive graphite, graphite felt sheets, carbon felt sheets, fabric having a carbon-covered electrolyte contact area, porous solid, filled with carbon, porous metal. 